The present invention relates to a process for improving the corrosion resistant characteristics of chrome plated aluminum and aluminum alloys.
Chrome may be plated onto aluminum in a variety of fashions. For instance, copper may first be plated on cleaned aluminum followed by the plating of a nickel layer upon which is finally plated chrome. Chrome may be plated directly upon a carefully cleaned aluminum surface. Many additional techniques have been proposed for chrome plating aluminum, two of the more common methods of surface preparation prior to chrome plating being the zincating and phosphoric anodizing process. The zincating process as shown in U.S. Pat. No. 1,627,900 involves depositing a thin layer of zinc by immersion of the aluminum article in sodium zincate solution. The anodizing process as shown in U.S. Pat. No. 1,947,981 requires the production of a thin porous anodic coating on an aluminum substrate by anodizing the aluminum article in aqueous phosphoric acid.
The corrosion resistance of aluminum and aluminum alloys treated by the aforesaid techniques is relatively poor, particularly under conditions of high chloride ion concentration. The coatings that have metallic layers between the chrome and the aluminum are prone to promote severe galvanic corrosion of the aluminum substrate at small pores in the outer chrome layer. This would normally be anticipated from the very active galvanic potential of aluminum, the noble potential of the chromium and many of the components in the intervening metallic layers. Somewhat surprising is the poor behavior of the barrier coating put on by anodizing aluminum and aluminum alloys in phosphoric acid. It would normally be expected that such a coating would have a significant protective effect against galvanic corrosion. Since this coating has extremely large pore diameters, it appears evident that any protection of the aluminum substrate is only provided by that thin barrier layer coating which is continuous. It appears evident that this thin barrier layer coating is adversely affected by the acid nature of the chrome plating bath.
The present invention contemplates a method of treating aluminum and aluminum alloys prior to plating which improves their corrosion resistant characteristics.
Accordingly, it is the principal object of this invention to provide an improved process for enhancing the corrosion resistant characteristics of chrome plated aluminum and aluminum alloys.
It is a further object of this invention to provide a process for pretreating aluminum and aluminum objects prior to plating which will produce a coating which is electronically conductive while still maintaining a coating of high ionic resistance.
It is still a further object of this invention to provide an effective and economical process for preparing aluminum and aluminum alloy surfaces which can be readily chrome plated by standard plating techniques.